The invention relates to a method for heating LowE glass panels in a tempering furnace provided with rollers, said method comprising a transfer of LowE glass panels on a conveyor constituted by rollers into the tempering furnace and then setting the discussed glass panels in an oscillating motion within the tempering furnace for the duration of a heating cycle, followed by delivering the discussed glass panels into a tempering station, and the glass panels being heated in the tempering furnace by means of top and bottom radiation heating elements, as well as by top and bottom convection heating elements.
The tempering process of certain types of LowE glass presents a problem in terms of tempering. The side edges and ends of glass deflect to form a bend towards the coating side, i.e. the edges bend upwards on an unloading table. Indeed, the phenomenon is referred to as a frame effect. The bend develops in glass at the downstream and upstream ends at a distance of about 70 mm from the glass edge and along the sides of the glass at a distance of about 40 mm.
The discussed phenomenon occurs mainly on LowE glasses manufactured with a so-called APCVD-method (ATMOSPHERE PRESSURE CHEMICAL VAPOR DEPOSITION), wherein the coating of glass is performed in a furnace atmosphere. On the other hand, in glasses manufactured basically pyrolytically by using a CVD-method (CHEMICAL VAPOR DEPOSITION) the discussed problem hardly exists.
The coating of glass made with an APCVD-method has a thermal expansion coefficient which appears to be higher than that obtained by a pyrolytically conducted CVD-method. As a consequence, the behaviour of glass in a tempering furnace is as follows:
1) In the early stages of heating, the glass tends to bend with its edges upwards as the LowE coating on top blocks the impact of heat radiation on the glass, the bottom side of the glass becoming hotter than the top side to result in cambering. (It is prior known to compensate for this cambering by using overhead convection blasting as disclosed in Patent publication U.S. Pat. No. 4,390,359).
2) In the middle stages of heating, the higher thermal expansion coefficient of the coating enables the convection blasting to be dramatically reduced or even stopped. At this point, the temperature difference between glass surfaces is compensated for by a heat expansion of the colder top-side coating, which is based on said higher thermal expansion coefficient.
3) In the final stages of heating, the temperature difference between top and bottom glass surfaces decreases, yet the oppositely directed bending effect caused by heat expansion of the coating keeps increasing, with the consequence that the glass tends to bend with its edges downwards, i.e. travels upon its edges. However, since the glass has softened at a tempering temperature, and especially since the glass edges tend to overheat as a result of three-dimensional heating occurring along the edge, the edge zone of glass yields like xe2x80x9cthe paw of a catxe2x80x9d, resulting in the abovementioned frame effect.
It is an object of the invention to provide a method capable of eliminating the occurrence of the above-mentioned frame effect.
This object is achieved on the basis of the characterizing features set forth in the appended claim 1. The non-independent claims disclose preferred embodiments of the invention.
Thus, according to the invention, it has been realized that, as the bottom side of a glass panel is subjected to powerful heating in the very final stage of a heating cycle, the bottom surface of the glass becomes hotter than the top surface, hence compensating for a cambering effect resulting from the higher thermal expansion coefficient of a LowE coating.
The prior known methods and furnace installations do not provide a solution to this problem, as apparent from the following review of patent publications describing the prior art.
U.S. Pat. No. 4,529,380 discloses a tempering furnace provided with convection blasting above and below a glass panel. According to the cited publication, the top convection or the bottom convection can also be used alone to provide a consistent temperature for maintaining the flatness of glass panels. The cited publication says nothing about the timing of top and bottom convection heating for various phases of a heating cycle. Neither does the cited publication discuss a problem regarding the heating of LowE glass panels, nor a discovery necessary for its solution, namely that the coating has a thermal expansion coefficient which is higher than that of the opposite surface of a glass panel. As a result of this, the sustenance of a consistent temperature between the top and bottom surfaces of a glass panel, as disclosed to be an objective in the cited publication, does not provide a solution to the discussed problem.
FI-1 00596discloses a heating method for glass panels, wherein the bottom surface of glass panels is heated with forced convection in the final stage of a heating cycle. This prior known method does not employ overhead convection, but, instead, the bottom section of a tempering furnace is cooled in the early stage of a heating cycle. In practice, this prior known method cannot successfully carry out the heating of LowE glass panels, since the coating is poor in terms of taking up radiation heat and, hence, the bottom cooling must be intensified to such a degree that the heating time becomes unreasonably long and the achievement of a temperature balance for a furnace becomes even otherwise more difficult, as the regulation of heating for a more slowly heating top surface can only be effected by radiation heat, which is substantially slower than convection heating in terms of adjustability.